Wales has become the first country in the UK to reduce speed limits in built-up areas from 30mph to 20mph by default - a move expected to save around 10 lives a year in the devolved nation.
The Labour Welsh government argued it is a vital road safety measure that will save lives and encourage active travel.
First minister Mark Drakeford said: 'It's going to take you a minute longer to make your journey, and we will save 10 people's lives in Wales every year as a result of that one-minute contribution - it doesn't seem an unfair bargain.'
The figures come from government analysis of the move, which states: 'The policy trades off a journey time cost of 1 min per journey against an average annual reduction of 9 fatalities, 98 serious injuries and 219 slight injuries, and an average annual increase in cycling and walking trips of around 11 million.'
Wales' 23 highway authorities were asked to consider which roads should remain at 30mph and these routes will have 30mph signs in place. An interactive map on DataMapWales shows which roads would stay at 30mph.
Prior to the order [Restricted Roads (20 mph Speed Limit) (Wales) Order 2022] coming into force on Sunday, around 2.5% of Welsh roads had a speed limit of 20mph. This has now increased to around 35%.
The order was brought in under the Road Traffic Regulation Act 1984, which allows for speed limits to be increased or reduced on restricted roads - where lamp-posts are no more than 200 yards (183m) apart.
The move is set against a background of stalled progress on road safety across the UK. In Wales, with no real progress since 2009 with the Government drastically missing 2020 casualty reduction targets.
In 2022, 1,014 people were killed or severely hurt on Welsh roads, more than 40% (421) were hit on a 30mph stretch of road.
A one-off cost of £32.5m has been attached to the roll-out, though ministers have insisted that this was more than 'outweighed' by reduced impact on the NHS and emergency services, which one study said could save £92m a year.
Overall, the economics suggest the policy could also create substantial wider economic benefits due to improved road safety (£1.4bn), environmental and health benefits from more active travel (£0.5bn) and further heretofore unquantified benefits from more vibrant and connected local economies.
The increased journey times disbenefits have been calculated at £6.4bn. This means a 'central estimate of the monetised net present value of the policy is calculated to be a negative £4.54bn'.
However, this is a contested figure not least because around three-quarters of the journey time disbenefits 'are likely to be attributable to trips with journey time impacts of less than 2 minutes,' government analysis suggests.
Sources at the Department for Transport told Highways that a more 'qualitative' analysis of time savings was needed to arrive at a more realistic figure rather than a linear understanding of time saved equals economic value.
From Sunday, the official punishment for doing more than 20mph in a built-up area in Wales would be a minimum £100 fine and three penalty points.
However, the police said the response will be proportionate and reasonable while drivers get used to the change. The focus for the first 12 months would be on education, with officers given a level of discretion.
The Welsh Conservatives have opposed the blanket rollout, calling it 'disastrous, frankly ludicrous and a war on motorists'.
The Conservatives said they were in favour of a targeted 20mph limit outside schools, hospitals and care homes.
